A new study suggests that non-native worms are eating up the forest floor, causing
sugar maples to die back and perhaps harming other forest dwellers.

The worms crawl in, the worms crawl out, and corpses aren’t the only casualties. A
Michigan Technological University scientist has fingered non-native earthworms as
a primary culprit in the decline of an iconic American tree.

An industry at risk

Sugar maples are prized as much for their valuable lumber as for their sugary sap
and dazzling fall colors. In Michigan alone, they are the basis of a multi-million-dollar
industry. But several years ago, foresters began noticing that the crowns of the big
trees appeared unhealthy, with bare limbs and little new growth.

“They were losing trees before they could harvest them,” said Tara Bal, a research
assistant professor of forest resources and environmental science. “We wondered what
was causing it.” Her findings were published July 26, 2017, in the journal Biological Invasions (doi.org/10.1007/s10530-017-1523-0 )

Drought was a suspect—the traditionally damp northern Great Lakes region was in the
midst of a dry spell. Other potential causes were forest management practices, soil
types, climate change and the mix of species in the area.

From 2009 to 2012, Bal made annual visits to more than 100 sites in Michigan’s Upper
Peninsula, northern Wisconsin and Minnesota. She found one factor that stood out:
the condition of the forest floor. And nothing affects a forest floor quite like earthworms.

Tara Bal examines the forest floor for signs of earthworm damage.

While they may seem as American as maple syrup, no earthworms—including the nightcrawler—are
native to the Upper Midwest. If there ever were any, the last ice age wiped them out
over 11,000 years ago. “All the earthworm species here are from Europe or Asia, brought
in when humans transplanted plants,” Bal said. Now, they and their egg cases hitch
rides on tires and muddy boots and spread when anglers dump bait in woods and waterways.
“It’s the same way invasive plants get around,” she said.

Voracious invaders

Before worms invade, the soil in a typical northern forest is blanketed by a thick
layer of leaf litter and other organic material. When worms arrive, they quickly gobble
it up and expose bare dirt. “And earthworms really like sugar maple leaves,” Ball
said. “They are sugary, soft, and have fewer tannins than other trees, like oaks.”

Sugar maples have a particular characteristic that makes them even more vulnerable
to earthworms. Ninety percent of their roots are in the top few inches of soil, so
the trees rely on the litter to keep the soil from drying out. With the litter gone,
maples can slowly die of thirst.

While the condition of the forest floor appears to be driving the maples’ dieback,
there could be other factors as well, Bal said. “At this point, we don’t really know
if earthworms are causing damage directly or making the soil and litter conditions
so poor that drought and other things are getting to the trees.”

Collateral damage: birds, bugs and more

Maples aren’t the only victims of these wriggly invaders. Many other forest creatures
depend on forest litter for their survival. “I was just looking at the health of the
trees, but earthworms are really affecting the whole forest,” Bal said. “You lose
wildflowers, young seedlings and many ferns.” The worms can reduce ground-nesting
birds, insects, amphibians and fungi whose lives are intertwined with the forest litter.
“I’ve never seen a salamander on the Michigan Tech Trails, but I’ve seen a lot of
nightcrawlers,” she noted.

People can slow their spread by throwing bait in the trash and washing mud off vehicles
and boots before traveling into the woods. “If you are a forest manager, you should
power-wash your equipment when you go from one place to another and use local road
fill as much as possible,” Bal said. “That’s a good practice for stopping the spread
of all exotic species, not just worms.”

It’s highly unlikely that the earthworms will go away, however. “Predictions are that
within 100 years, 95 percent of our sugar maple forests will be invaded by earthworms,
and there’s no worm-icide.”

Fewer maples, more diversity

“This means change for the forest,” said Bal. “Forest managers will have to start
thinking outside the box to keep forests and trees healthy and regenerating.”

That could mean a return to diversity; sugar maples did not always dominate northern
hardwood forests. “We have pure maple stands because it’s a valuable wood,” she said.
“If some of the sugar maple dies back, you could have the return of other species,
like basswood, birches and ironwood.

“That could make the forest more resilient,” she suggested. “The way things stand
now, if something that loves sugar maple, like the Asian longhorn beetle, ever got
here, we could lose our hardwood timber industry.”

The study was conducted in the Hiawatha, Ottawa, Superior and Chequamegon-Nicolet
National Forests; Baraga State Forest; and commercial forestland owned by the Boston-based
investment firm GMO. Funding was provided by GMO and the USDA Forest Service—Forest
Health Monitoring Program (Project NC-EM-B-10-02).

The article on the study, “Evidence of Damage from Exotic Invasive Earthworm Activity
Was Highly Correlated to Sugar Maple Dieback in the Upper Great Lakes Region,” was
published July 26, 2017, in the journal Biological Invasions (doi.org/10.1007/s10530-017-1523-0). Coauthors are Bal, Professor Andrew Storer and
Research Professor Marty Jurgensen, all of Michigan Tech’s School of Forest Resources
and Environmental Science.

Michigan Technological University is a public research university, home to more than
7,000 students from 54 countries. Founded in 1885, the University offers more than
120 undergraduate and graduate degree programs in science and technology, engineering,
forestry, business and economics, health professions, humanities, mathematics, and
social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway
and is just a few miles from Lake Superior.